The present invention relates to fluid flow measurement and, more particularly, addresses overcoming inaccuracies in flow measurement. The invention provides a method for eliminating errors in mass and volumetric (and energy) flow rates measured for primarily gaseous fluid with respect to temperature changes in the gaseous fluids.
Flow sensors that utilize a pair of thin film heat sensors and a thin film heater are known. An example of such a device is illustrated and described in U.S. Pat. No. 4,501,144 to Higashi, et al. The thin film heat sensors may be arranged in a Wheatstone bridge so that the output of the bridge is representative of flow. These microanemometers or "microbridges" are produced using similar techniques to those used for producing integrated circuits and are thus quite inexpensive.
As will be described in greater detail herein, such microanemometers or microbridges are capable of quite accurate flow sensing when directly exposed to a stream of fluid flowing past them. In this manner, such a sensor can be used to directly measure the flow velocity of the fluid.
While such a sensing system can be used to approximately measure mass flow, a significant level of error has been experienced with respect to changes in composition of the measured fluid in prior devices using this system. The above referred to U.S. Pat. No. 4,961,348 describes in detail a method of correcting gaseous fluid flow measurement for changes in composition. Applying the composition corrections provided in U.S. Pat. No. 4,961,348 will generally provide flow measurements accurate to within 10%. While this accuracy is sufficient for many nonprecision applications, there remain many precision applications that require a greater accuracy. Further investigation has resulted in the discovery of the algorithm of the present invention. The investigation showed that after applying the composition correction, there still remained an error due to the temperature difference of the gaseous fluid relative to the temperature of the calibration fluid.
For illustration purposes, errors due to temperature of about 0.2% per .degree. C. were observed when the temperature of the gaseous fluid varied about .+-.10.degree. from the 23.degree. C. calibration temperature. This error of approximately 5% for a 10.degree. temperature difference is unacceptable for many precision gas flow measurement applications. Thus a need exists for a method to correct gas flow measurements for variations in the temperature of the flowing gas from the temperature of the calibration gas.